VPN strange problem

My MT RouterOS box have 2 nic, 1 for public and another for internal

i have 8 public ip and I assign all public ip on public nic, ex. 204.x.y.2 ~ 204.x.y.9

i tried to set up pptp server, i can only use the first public IP 204.x.y.2 to accept pptp dial in successfully, but fail on other public ip ------ when user pptp-in, winbox show user connected, but after disconnect after 0.5second, then user winxp said vpn client error 619.

i do not know why this strange problem, because i do not think it is not the matter of encryption or even firewall rule, since users pptp-in could connect sucess to my first IP, although pptp-in fail on other IP, user could reach my box but just disconnect very soon.

any idea? thx a lot!